The plugin does not correctly check decoder profiles, so it should
really not be enabled automatically. It can still be enabled manually
from the UI or CLI as before.

Only MPEG1/2 slice threading is currently known as broken.

This rectifies commit df252eac.

Regression from b71c85b3.

The first show() was never satisfied because the widget was not
initialized at this point.

Fix #6375 and may benefit to #8936

This is untested due to lack of compatible hardware.

It seems impossible to describe this chroma with
vlc_chroma_description_t. Thus picture allocation will fail.

This is untested due to lack of adequate hardware.